It sounds like you're interested in a dating technique that involves comparing fossils found in different rock layers. This is actually the core of a very important dating method in geology and paleontology called biostratigraphy.

Here's how it works:

1. Fossil Index:

* Some fossils are particularly useful for dating. These are called index fossils. They have specific characteristics:

* Wide distribution: They were found across a large geographic area.

* Short lifespan: They lived for a relatively short period of time.

* Distinct features: They are easily recognizable and distinguishable from other fossils.

2. Rock Layers:

* Rock layers (strata) are formed over time, with older layers at the bottom and younger layers on top. This is the principle of superposition.

3. Correlation:

* By comparing the presence and absence of index fossils in different rock layers, scientists can correlate layers across different locations. If two rock layers contain the same index fossils, they likely formed during the same period of time.

4. Dating:

* Index fossils are like "time markers" in the rock record. The presence of a specific index fossil tells us the approximate age of the rock layer where it was found.

Example:

Let's say you find a trilobite fossil in a rock layer. Trilobites were common in the Paleozoic Era, and different species lived for specific periods within that era. If you can identify the trilobite species, you can narrow down the age of the rock layer to a specific time period within the Paleozoic Era.

Limitations:

* This method isn't perfect. The fossil record is not complete, and some species might have existed for longer periods than others.

* Dating relies on the availability of accurate index fossils and thorough research on their lifespans and distribution.

Overall:

Biostratigraphy is a powerful tool for understanding the relative age of rocks and fossils, contributing significantly to our knowledge of Earth's history and the evolution of life.